1. Field of the Invention
The present invention refers to an electrically operated dispensing appliance in which the electromechanical drive assembly comprises a motor actuating at least one driving screw for the slow advance and the relieving stroke as well as coupling means allowing to switch to fast retraction and fast forward feed.
High pressure forces are required in order to press the compositions contained in containers or cartridges through a mixing device, e.g. a static mixer, and the cartridges, which are generally made of synthetic materials, are expanded by the resulting load. This cartridge pressure must be instantly relieved as soon as the dispensing stops, or else the compositions will continue to flow out of the mixer, the so-called afterflow. To prevent this, the thrust members are capable of performing a small relieving stroke just after the dispensing motion. It is further required that the retraction of the drive assembly or the forward feed of the thrust members into a partly dispensed cartridge be effected substantially faster than the dispensing motion itself.
2. Description of the Prior Art
U.S. Pat. No. 5,203,476 takes this problem into account by effecting the relieving stroke through a mechanical axial displacement of the driving screws by means of a toggle joint. According to U.S. Pat. No. 5,464,128, both the relieving stroke and the fast retraction are effected by a second motor and corresponding magnetic couplings.
It is further known from European Patent No. 0,492,413 to use a magnet coupling in order to disengage the advance drive from a chain driven thrust slide to allow a certain cartridge relief and a manual retraction. In this case, the fast forward feed is effected manually by means of a handle.
In the electrically operated dispensing appliances of the prior art discussed above, the electric drive has several drawbacks. These drawbacks have been eliminated by a drive disclosed in the co-pending U.S. application Ser. No. 09/310,919, where two motors are used in order to operate the dispensing motion and the relieving stroke, on one hand, and the fast forward feed and the fast retraction, on the other hand. It has been found that this concept can be improved, so as to save the costs for the second motor, in particular, and to reduce the noise caused by the second motor.
Electrically operated dispensing appliances are already known from the previously mentioned U.S. Pat. Nos. 5,203,476 and 5,464,128 to the same Applicant. These appliances are essentially so designed that a driving screw advances a slide to which two thrust rods are fastened, and the compositions are dispensed by means of two cartridge pistons.
In another known appliance according to European Patent No. 0,492,413, the slide is advanced by two laterally disposed chains and, in another case, by toothed racks. These known devices have fundamental disadvantages. The different reaction forces that are generated at different dispensing ratios load the thrust rods with high torque and cause important canting friction and deformation forces on the guiding bridge and on the necessary guiding elements, whereby such appliances are expensive in construction and difficult to lubricate and seal.
In other cases, e.g. according to U.S. Pat. No. 4,180,187, where the driving screws must be lubricated due to the high dispensing forces are directly introduced into the cartridges, these driving members are subject to soiling and difficult to clean if soiled by the dispensed component materials, which is impractical for the user. If the guiding bridge is axially moved by means of chains, an essential drawback is that the very slow dispensing speed requires a gear having a high reduction ratio as well as a very costly high torque magnet coupling allowing to uncouple the drive unit for the relieving stroke and retraction.
On the background of this prior art, it is the object of the present invention to eliminate the drawbacks discussed above and to provide an economical and silent drive assembly. This object is attained by an electric drive assembly wherein a gear motor is connected to the main shaft by a first gear and a first shifting sleeve in order to operate the advance and the relieving stroke at high loads, whereas for the operation at smaller loads, i.e. fast retraction and fast forward feed, it is connected to the main shaft by a second gear, a driving shaft, and a second shifting sleeve, and wherein an electromagnet is connected to said two shifting sleeves in such a manner that one or the other shifting sleeve is selectively connectable to the main shaft.
The dependent claims refer to developments of the electric drive assembly of the invention and also to an advance unit which is used as a mechanical structural assembly in dispensing appliances for multiple component compositions.